1. Field of the Invention
The present invention relates to surface acoustic wave filters having improved characteristics outside the pass band.
2. Description of the Related Art
Surface acoustic wave filters for use in a narrow band and diminished in insertion loss are already known which comprise two filters of the transversely coupled mode resonator type connected in cascade and each comprising an interdigital electrode (interdigital transducer) and two reflectors arranged respectively at opposite sides of the electrode. Such surface acoustic wave filters are used chiefly as IF (intermediate-frequency wave) filters in communications devices.
Sufficient attenuation outside the pass band is required of IF filters, while when the filter is composed of filters of the transversely coupled mode resonator type connected in cascade and identical in characteristics, the filter in the first stage and the filter in the second stage are also approximately identical in characteristics outside the pass band, therefore permitting occurrence of spurious resonance at approximately the same frequency and consequently giving rise to the problem that spurious resonance occurs at a high level outside the pass band.
To solve this problem, two cascaded component filters are made different in the pitch of reflectors and/or the number of pairs thereof while being made identical in characteristics in the pass band, and are thereby made different in characteristics outside the pass band as disclosed, for example, in JP-A No. 131291/1995. With this arrangement, the first-stage filter and the second-stage filter differ in frequency at which spurious resonance occurs, whereby the ratio of the spurious resonance level to the signal level in the pass band is decreased, consequently affording a surface acoustic wave filter which is improved in characteristics outside the pass band.
Different characteristics are made available outside the pass band generally by making the component filters different in the pitch of reflectors or the number of pairs thereof as stated above, or in the number of electrode digits of the interdigital electrode. However, with filters of ever decreasing sizes developed in recent years, the space available for providing electrodes becomes limited, making it no longer possible to use such a method. It is therefore desired to provide a novel method of improving the characteristics outside the pass band.
Although the techniques disclosed in the above publication, JP-A No. 131291/1995, afford improved characteristics outside the pass band, the improvement is limited only to the spurious resonance occurring at the higher frequency side outside the pass band since the component filters are made different in the pitch of reflectors and/or the number of pairs thereof and thus made to differ in characteristics outside the pass band.